Certain state-of-the-art laser printers presently utilize power supplies composed of a switching power supply combined with an auxiliary power supply. The switching power supply is used to provide the primary direct current (DC) voltages that the laser printer operates on. The switching power supply provides three operating voltages for the printer, +5 volts, +12 volts, and +24 volts. These are standard voltages with +5 volts used for the control computer, +12 volts used for analog circuitry, and +24 volts used for powering cooling fans, electric motors, electromechanical relays, and solenoids. The auxiliary power supply provides +22 volts and is used to power exhaust fans after the switching power supply has been turned off. The exhaust fans are powered for a short time after the printer has been turned off to vent ozone from the photo conducting print drum. Ozone generated from the printing process attacks the outer stirface of the photo conductor and decreases the useful life of the photo conductor. An electronic time delay senses loss of 24 volts DC and starts timing to allow the auxiliary power supply to operate for approximately three minutes to ensure that the ozone is sufficiently vented from the photo conductor.
The use of two power supplies to perform the function of supplying primary switching power and auxiliary power is very expensive. An auxiliary power supply utilizing +120 v AC or +240 v AC and supplying +22 v DC requires a bulky transformer.
The auxiliary power supply is always connected to the AC line which results in constant power consumption when the printed is turned off and higher power consumption when the printer is turned on. This higher power consumption results in higher operating temperatures within the printer power supplies, thereby requiring an extra 2-watt DC power supply cooling fan. The power consumption of the auxiliary power supply when plugged into 120 v AC is about 2 watts. Future regulatory requirements, both voluntary and non-voluntary will require a lower "turned off" power consumption.
The prior art method of using two power supplies, one for supplying operating power for the printer and a second for supplying auxiliary power to exhaust fans when the printer is turned off, requires a much larger and expensive power supply assembly. Space is at a premium in commodity grade laser printers so eliminating the secondary power supply reduces the cost of the power supply assembly and reduces the space, and its associated cost, that the power supply must reside in. This prior art also mandates the need for the extra DC power supply cooling fan.
It is, therefore, the purpose of this invention to: eliminate the auxiliary power supply, eliminate the DC power supply cooling fan, reduce the number of electronic devices, reduce the space required by the power supply, reduce power consumption, and reduce power supply assembly costs.